The present invention relates to a hydraulic control system of an automatic transmission for a vehicle and a shifting control method thereof. More particularly, the present invention relates to a hydraulic control system and a shifting control method which improves shift response, controls shift modes according to engine power ON/OFF states, and controls at least one input element independently, and to a shifting control method using such a hydraulic control system.
Generally, a conventional automatic transmission includes a torque convertor, a multi-stage gear shift mechanism connected to the torque convertor, and a hydraulic control system which selects a gear stage of the gear shift mechanism according to a driving state of the vehicle.
The above hydraulic control system includes a pressure regulator, which regulates hydraulic pressure created in a oil pump; manual and automatic shift controllers, which select a shift mode; a hydraulic pressure controller, which controls shift quality and shift response for smoothly selecting a shift mode during shifting; a damper clutch controller, for operating a torque convertor damper clutch; and a hydraulic pressure distributor, which supplies an appropriate amount of hydraulic pressure to each of the friction elements.
The amount of hydraulic pressure supplied by the hydraulic pressure controller, and the hydraulic pressure control method greatly affects shift quality and shift response. Accordingly, when down shifting from fourth speed to third speed, the hydraulic pressure, which is supplied to an input element of the fourth speed, is reduced temporarily, resulting in engine speed run-up and shift shock.
As shift control is performed, whether the engine is in a power ON or OFF state, the amount that the driving force can be increased in the power OFF state is limited. As the length of time that the transmission stays in fourth speed is reduced, a transmission tie-up phenomenon occurs. In addition, when skip down shifting from fourth speed to second speed, shift response is slow because the transmission passes through third gear. Also, when driver-induced skip shifting (e.g., by suddenly and briefly releasing the accelerator pedal) from second speed to fourth speed is performed, shift response is again slow because the transmission passes through third speed. This is because the conventional hydraulic control system uses the same hydraulic pressure as both a release pressure for friction elements in fourth speed and operating pressure for friction elements in third speed, whereby the problem of passing through third speed results. Because of this problem, especially when shifting from third speed to fourth speed, because hydraulic pressure lines to the friction elements of the third speed and fourth speed are interconnected, independent control of the friction elements is needed.